1. Field of the Invention
This invention relates generally to optical reflective scanning systems and more particularly to methods and apparatus for suppressing the deleterious effects of surface light scatter in an optical reflective scanning system.
2. Description of the Prior Art
Optical reflective scanning systems are used in many types of devices, such as photo copiers and computer input periferals, to scan the surface of a document or other item and to produce a digital (black or white) representation therefrom.
There are three major types of reflective scanning systems now in use. In a first type of system, a light source flood illuminates the surface and a detector then sequentially scans the entire surface to detect the amount of light reflected from small areas along the scan path. The light reflected from these small areas is then used to generate digital signals which indicate whether the respective areas are predominantly black or white. From these digital signals an image of the scanned surface can be reproduced or stored.
A second type of system includes a scanning light source which sequentially illuminates small areas of the surface to be scanned and a wide angle detector and associated electronics which detect light reflected from the entire surface and develops digital signals that indicate whether small areas along the scanned path are predominantly black or white.
The third type of system is basically a combination of the first and second system types in that it includes a scanning means for spot illuminating small areas along the scanned surface and a scanning spot detector which follows the scanning beam and detects light reflected from the illuminated surface. The third type of system has the best resolution but is more complex and thus generally more expensive than the other two systems. Furthermore, the resolution of the optical scanning device increases with a decrease in the area that is "spot" illuminated by the light source until the area becomes so small that the so-called "scatter effect" comes into effect. Scatter effect refers to conditions wherein light is "scattered" from a directly illuminated area to a surrounding area and is then rescattered back into the directly illuminated area so as to have the effect of increasing the intensity of the light reflected from the directly illuminated area.
A problem encountered in the use of reflective scanners found in the prior art is that the quantity of light reflected from the illuminated area is partially dependent upon the amount of scattered light returned from the surrounding areas. For example, a white spot on a surface surrounded by a dark area reflects less light than a white spot surrounded by a white area. As a result a grey spot might be detected as black when surrounded by a dark area and as white when surrounded by a light area.